These studies are concerned with endocrine influences on immune function, hematopoiesis and lymphoproliferative disease. The pituitary dwarf mouse offers a unique model to study the relationship of growth hormone to the immunohematopoietic systems. Analysis of the effects of milk on hematopoietic stem cell potential may provide insight into maternal influences of hematopoiesis in the newborn. Primary attention will be focused on (a) the effect of milk on the recovery of the immune system following sublethal irradiation or cortisone treatment. Special emphasis will be placed on the thymus dependent immunocompetence, the immune responsiveness to sheep erythrocytes and evaluation of hematopoietic recovery measured by the CFU assay and radiolabeled iron uptake. (b) Effect of growth hormone on immunocompetence during first week of life with emphasis on evaluation of T-cell function measured by mitogen responsiveness and graft-versus-host reactivity. (c) Induction of lymphoproliferative disorders in dwarf mice with oncogenic viruses and administration of chemical carcinogens. (d) Studies of immunocompetence of dwarf lymphocytes measured by their reactivity as stimulator cells of responder cells in the mixed leukocyte culture reaction.